Generally, a gas combustion type driving tool has a cylinder, a piston, and a cylinder head disposed above the cylinder to make up a fastener driving mechanism, which are formed of metal such as aluminum, and the cylinder and the cylinder head are fixed to a body made of synthetic resin. That is, the cylinder head is fixed to an upper end of the body, and the cylinder is fixed to an intermediate part and a lower portion of the body. A movable sleeve is disposed to be movable in a vertical direction between the cylinder and the cylinder head, and a combustion chamber is formed therein, and opened or closed by the movable sleeve (refer to patent document 1).
Patent document 1: JP-A-2005-329533
In the above gas combustion type driving tool, the combustion chamber is placed in a tightly closed state by moving the movable sleeve upward. A fuel gas is supplied into the tightly closed combustion chamber, and agitated and mixed with the air by a central rotating fan to obtain a mixed gas. The mixed gas is ignited and explosively burned by an ignition plug. The piston within the cylinder is driven by the combustion pressure, and a fastener is driven from a nose portion provided on a lower portion of a tool main body by a driver integrally connected with the piston.
However, there is the following problem with the constitution in which the cylinder and the cylinder head made of metal are fixed to the body made of synthetic resin. That is, when the mixed gas is explosively burned within the combustion chamber at the time of driving, a temperature within the combustion chamber rises to considerably high temperature, and the cylinder head is subjected to a great pulling force upward and the cylinder is subjected to a great pulling force downward owing to the combustion pressure. Also, when the piston is driven to hit against a bumper at a lower end of the cylinder, a great inertial force acts on the body. In this manner, because of such a structure that a force applied on the cylinder head or cylinder at the time of driving is received by the body made of synthetic resin, a rigidity is required for the body. Further, the body made of synthetic resin is easily deformable when subjected to heat or external force, changing the relative positional dimension between the cylinder and the cylinder head fixed to the body, so that a seal failure or an up and down operation failure of the combustion chamber is caused. Therefore, it is required to increase the thickness of the body, which causes the weight to increase.
Also, an upper housing 50 making up an upper portion of the body is sufficient if there is a space for disposing a fan motor, an ignition plug and an exhaust nozzle, as shown in the fore side of FIG. 4, similarly to the cylinder head. However, a square garnish rim 54 for connecting via a bridge 51 and an annular portion 53 to a lower housing 55 must be provided outside the required space. As a stress acting on the central part of the cylinder heat owing to a combustion pressure or inertial force is concentrated on the bridge 51, it is required that there is an enough volume as the thickness to secure the strength of the bridge 51. Therefore, there was a problem that the total weight was increased.